It has been known for many years that a color image can be produced by means of a reaction involving contact between an electron-donating or proton-accepting colorless organic compound (hereinafter referred to as a color former) and an electron-accepting or proton-releasing solid acid (hereinafter referred to as a color developer). This phenomenon is embodied in pressure-sensitive paper as described in U.S. Pats. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, 3,418,250 and 3,672,935. A printing method has been proposed that involves preparing a sheet coated with color developer and producing a color image on that sheet by supplying an ink containing a color former; this technique is disclosed in German Patent Application (OLS) No. 1,939,962.
The color developer has the properties defined above and is selected from among clays, phenolic resins, and metal salts of aromatic carboxylic acids. Since these color developers are usually coated in a uniform thickness on the entire surface of a support, the non-image areas of the color developer sheet are desensitized by printing or otherwise coating a composition containing an appropriate desensitizer.
Desensitizer are described in U.S. Pats. Nos. 2,777,780, 3,890,156, 3,931,430, 3,952,117, 4,012,538, 4,022,624 and 4,101,690; West German Pat No. 2,526,592; West German Patent Application (OLS) Nos. 2,359,079 and 2,727,194; Belgian Pat. No. 804,221; Japanese Patent Publication Nos. 29546/71, 23850/74, 14571/75 and 29365/75; and Japanese Patent Application (OPI) Nos. 125018/77 and 67291/81 (the term "OPI" as used herein means a "published unexamined Japanese Patent Application"). Specific examples of the desensitizer include dodecyl trimethylammonium chloride, dodecylamine, 2,4,4-trimethyl-2-oxazoline, xylenediamine, polyoxyethylene alkylamine, polyoxyethylene alkylether, polyoxyethylene alkylphenyl ether, polyethylene glycol, polypropylene glycol, glycidyl ether adducts of amines, etc.
These desensitizers, however, are not completely satisfactory in their desensitizing effects and their effectiveness is particularly low with respect to such fluoran color formers as 3,6-bis-diphenylaminofluoran and 3-diethylamino-7-dibenzylaminofluoran. If a color former is brought into contact with a color developer sheet after a desensitizer is coated in the non-image areas, the non-image areas initially seem to be completely desensitized. However, if the sheet is exposed to light (particularly sunlight) a color image often appears on the non-image areas. In order to avoid this problem, a very thick coat of the desensitizer must be formed on the sheet of color developer by printing, but then the printed surface will dry so slowly that the printing speed cannot be increased to an industrially acceptable level.
In addition, if characters are written or printed with a color ink on the surface of the sheet of color developer that has been coated with an increased amount of desensitizer, the resulting ink image wil undergo extensive discoloration or may be blurred.